Group members: Sam Windross Morgan Vondee Jaynita Patel Sara Khan Raj Piraveen Rajaratnam
Introduction
Polymorphism is essentially a condition where different phenotypes exist in the same population of a species; on a genetic level it is where many alleles are at the same locus of a chromosome in a population (1). When new alleles arrive in a population through mutation, a number of forces are involved in either fixing them (in the population) or causing them to be lost completely. These forces are gene flow, genetic drift and natural selection. The investigation studies whether natural selection and genetic drift or genetic drift alone is responsible for the polymorphism exhibited in the snail species Cepaea nemoralis; the phenotypic polymorphism that will be studied is the banding and colours of the C. nemoralis snail shell. Cepaea nemoralis, commonly known as the brown-lipped snail or the banded wood snail, was discovered by Linnaeus in 1758. The snails are native to Western Europe but are generally distributed in North America and Central and Western Europe, and are commonly found in urban areas, but can also be found in woodland, shrubland and grassland areas (1). Cepaea nemoralis are studied due to their varied phenotypes such as different shell colours and different banding patterns. They are preferable to study because they only move up to approximately 30 meters in their lifetime, compared to humans, who have a greater ability to travel (high rates of gene flow) as well as a greater spread across the planet. C. nemoralis have a short life span meaning that the phenotypes of several generations can be analysed. To identify the effects of Natural Selection, Genetic Drift and Gene Flow within a population, C. nemoralis will be our organism of choice. The sampling strategy involves taking samples along a transect from shrub habitat and grassland; these samples will be taken on the same altitude. By ensuring the same altitude for each sampling area we will control factors caused by varied elevation such as temperature. The samples will be around 20 meters apart allowing us to attain a large snail sample that is representative of the population, as well as reduce the possibility of gene flow between the different areas. If we were to determine that polymorphism of C. nemoralis was due to natural selection we would observe a phenotype that has a significantly higher frequency in one type of habitat than another. If the intensity of genetic drift was greater than selection on the population of snails, we will expect to see random phenotypic frequencies across the different habitats, also there would be smaller differences in the frequency of polymorphs between populations (2). Whether a pattern indicating natural selection or genetic drift (as the cause of polymorphism) is identified several replicate samples will be taken to ensure the consistency of the observation. A chi squared test will be used to assess the significance of the results.
In this investigation the null hypothesis is that polymorphism of Cepaea nemoralis in grass land and shrubs is not due to natural selection, but genetic drift and gene flow. The alternative hypothesis is that polymorphism of Cepaea nemoralis in grassland and shrubs is due to natural selection in the majority of cases, with genetic drift and gene flow being minor influences. We hope as stated in our hypothesis to see how selection acts within the snail population as opposed to that of drift and gene flow, by observing consistent phenotypic frequency variations between the two habitats of shrub and grass land.
1.Jones, J.S., Leith, B.H., Rawlings, P. 1977. Polymorphism in Cepaea: a problem with too many solutions?Ann. Rev. Ecol. Syst.8:109-143.
2. Barton, N.H., Briggs, D.E.G., Eisen, J.A., Goldstein, D.B., Patel, N.H. 2007. Evolution. Cold spring Harbor Laboratory press. New York, united states of America.
Group members:Sam Windross
Morgan Vondee
Jaynita Patel
Sara Khan
Raj Piraveen Rajaratnam
Introduction
Polymorphism is essentially a condition where different phenotypes exist in the same population of a species; on a genetic level it is where many alleles are at the same locus of a chromosome in a population (1). When new alleles arrive in a population through mutation, a number of forces are involved in either fixing them (in the population) or causing them to be lost completely. These forces are gene flow, genetic drift and natural selection. The investigation studies whether natural selection and genetic drift or genetic drift alone is responsible for the polymorphism exhibited in the snail species Cepaea nemoralis; the phenotypic polymorphism that will be studied is the banding and colours of the C. nemoralis snail shell.
Cepaea nemoralis, commonly known as the brown-lipped snail or the banded wood snail, was discovered by Linnaeus in 1758. The snails are native to Western Europe but are generally distributed in North America and Central and Western Europe, and are commonly found in urban areas, but can also be found in woodland, shrubland and grassland areas (1). Cepaea nemoralis are studied due to their varied phenotypes such as different shell colours and different banding patterns. They are preferable to study because they only move up to approximately 30 meters in their lifetime, compared to humans, who have a greater ability to travel (high rates of gene flow) as well as a greater spread across the planet. C. nemoralis have a short life span meaning that the phenotypes of several generations can be analysed.
To identify the effects of Natural Selection, Genetic Drift and Gene Flow within a population, C. nemoralis will be our organism of choice. The sampling strategy involves taking samples along a transect from shrub habitat and grassland; these samples will be taken on the same altitude. By ensuring the same altitude for each sampling area we will control factors caused by varied elevation such as temperature. The samples will be around 20 meters apart allowing us to attain a large snail sample that is representative of the population, as well as reduce the possibility of gene flow between the different areas.
If we were to determine that polymorphism of C. nemoralis was due to natural selection we would observe a phenotype that has a significantly higher frequency in one type of habitat than another. If the intensity of genetic drift was greater than selection on the population of snails, we will expect to see random phenotypic frequencies across the different habitats, also there would be smaller differences in the frequency of polymorphs between populations (2). Whether a pattern indicating natural selection or genetic drift (as the cause of polymorphism) is identified several replicate samples will be taken to ensure the consistency of the observation. A chi squared test will be used to assess the significance of the results.
In this investigation the null hypothesis is that polymorphism of Cepaea nemoralis in grass land and shrubs is not due to natural selection, but genetic drift and gene flow. The alternative hypothesis is that polymorphism of Cepaea nemoralis
in grassland and shrubs is due to natural selection in the majority of cases, with genetic drift and gene flow being minor influences. We hope as stated in our hypothesis to see how selection acts within the snail population as opposed to that of drift and gene flow, by observing consistent phenotypic frequency variations between the two habitats of shrub and grass land.